after the workstation has been locked for many hours, I cannot unlock it
anymore. Only the background and mouse cursor is visible, no login
screen.. nothing. Doesn't respond to ctrl-alt-del.

for a shorter locking time, e.g one hour, it seems work fine.

Any ideas? Using XP SP1, 4.9SP2 patch D.
I noticed few posts regarding to LDAP contextless logins.. I have that
disabled as I still use old DSCAT catalogs.

Hibernate and such features are disabled from XP.